From declining annual sea ice extent to greening tundra, environmental change has been gradually unfolding in the Arctic for decades. However, some changes have come more abruptly.

Satellite, aerial and ground surveys spanning more than 1,000 kilometers (600 miles) along Alaska’s Brooks Range have observed stream water changing from clear to orange in more than 200 watersheds. What’s more, scientists are finding that the change has largely occurred over the past 10 to 12 years, coinciding with a sharp rise in air and soil temperatures.

Thawing permafrost soils, accelerated by warming air and soil temperatures, is the most likely cause of “rusty” rivers, scientists say. They assume that water is now finding thawed soil and bedrock where it had not before. Chemical erosion of minerals leaches iron, sulfuric acid and trace metals into streams, similar to the process behind acid mine drainage, which similarly contaminates and discolors water near abandoned mines. Microbes can also contribute to color change by producing a soluble form of iron as they digest plant and animal matter in thawing soils, which is then oxygenated or “oxidized” in streams.

Only recently have researchers begun to understand the prevalence of river oxidation in Arctic regions. In 2024, a team from the National Park Service, the U.S. Geological Survey, and university scientists documented 75 northern Alaska streams that recently changed from clear to orange. With further exploration, mostly using high-resolution satellite images, they added 200 more observations. The locations of these discolored currents, published in NOAA’s Arctic Report 2025, are shown on the map above.

“I’m still surprised by the wide spatial scope of our observations,” said Brett Poulin, an environmental toxicologist at the University of California, Davis. He and his collaborators have been monitoring the region’s streams since 2013, when many were still clean. “We are now seeing hundreds of streams that have changed color seemingly overnight, even in corridors designated as National Wild and Scenic Rivers,” he said.

Observations from NASA/USGS Landsat satellites allowed the team to determine the timing of several of these changes. For the 2024 study led by National Park Service ecologist Jon O’Donnell, the team calculated a redness index based on red and blue spectral information sensitive to the color of iron hydroxides (i.e., rust) in the water. After analyzing a subset of streams, they found that some became rusty around 2018 and stayed that way, while others had periods of rust and then became clear again.

One stream that underwent a sudden change is the Agashashok River in the Noatak National Reserve (above). In 2019, an increase in reddening values ​​appeared in Landsat data along this waterway. Ground and aerial surveys the same year found an orange section of the river several kilometers long, and the vegetation around nearby seeps and groundwater springs appeared blackened. “The Landsat archive has proven exceptionally useful for investigating the historical occurrence of river rust where streams and rivers are large enough,” Poulin said.

Having obtained a better picture of the extent and timing of the phenomenon, researchers want to focus on the conditions that drive the appearance of the orange color and the annual and seasonal changes. Deep snow cover can play a role some years, for example, by insulating the ground from cold winter temperatures and allowing permafrost to thaw in early summer. Additionally, periods of higher flow throughout the year can dilute discoloration. The team is planning a geophysical survey along a hillside where acidic groundwater is discharged to the surface to investigate subsurface geology, hydrology and permafrost.

In addition, they seek to quantify the effects on water quality and aquatic ecosystems. Communities depend on these river systems for drinking water and subsistence fishing, and a decline in stream biodiversity has already been documented in some locations coinciding with the orange color of the water. Researchers are now delving into patterns of toxicity over time and space, such as where rusty rivers overlap with known spawning areas of migratory fish.

“The river oxidation phenomenon is a good example of an unforeseen consequence of thawing permafrost in the Arctic,” Poulin said. “Furthermore, it is consistent with the occurrence of acid rock drainage following the loss of the cryosphere throughout the Earth.”

NASA Earth Observatory images taken by Michala Garrison, using stream location data from O’Donnell, JA, et al., and Landsat data from US Geological Survey. Lindsey Doermann Story.

• NASA Earth Observatory (2024, January 16) Rusty Rivers. Accessed July 9, 2026.
• O’Donnell, J.A., et al. (2025) Rusty rivers: assessing causes and consequences in Alaska and across the Arctic. Arctic Report Card 2025.
• O’Donnell, J.A., et al. (2024) Metal mobilization from thawing permafrost into aquatic ecosystems is causing oxidation of Arctic streams. Earth and Environment Communications5, 268.
• US Geological Survey (February 27, 2026) Oxidation of Arctic Rivers: Freshwater ecosystems respond to rapid metal uptake. Accessed July 9, 2026.